1. Field of the Invention
The present invention relates, in general, to an apparatus for evaluating the performance of a fuel cell which generates electricity using an electrochemical reaction between externally supplied fuel and air, and, more particularly, to a cell or stack for evaluating the performance of a fuel cell and a method of evaluating the performance of a fuel cell using the cell or stack, which are suitable for evaluating the performance of a fuel cell in an environment in which a uniform temperature is maintained.
2. Description of the Related Art
Generally, technology for a fuel cell, which is a device for directly converting the energy of fuel into electrical energy, is well known. This fuel cell is typically implemented such that an anode and a cathod are attached to both sides of a polymer electrolyte membrane, and is operated such that the electrochemical oxidation of hydrogen, which is fuel, occurs on the anode (oxidation electrode or fuel electrode), and the electrochemical reduction of oxygen, which is an oxidizer, occurs on the cathode (reduction electrode or air electrode). By these reactions, the transportation of electrons from anode to cathode side occurs and generates the electricity.
FIG. 1 is a diagram schematically showing a conventional fuel cell in which a temperature control device using a heater is installed.
As shown in FIG. 1, a fuel cell system 1 includes a solid polymer-type stack 10 for generating electricity, a reformer 11 for producing hydrogen gas to be supplied to the stack 10, an air line 12 for supplying air to the stack 10, a battery 13 for charging or discharging the electricity generated in the stack 10, a temperature detection means for detecting the temperature of the stack 10, a heating means for applying heat to the stack 10 when the temperature detected by the temperature detection means is below a specified value, a control unit 14 for controlling various types of valves and driving operation, and a discharge line (not shown) for discharging H2O generated in the stack 10.
The stack 10 can be implemented in the form of a single unit cell in which an electrochemical reaction occurs, or a structure in which unit cells are sequentially stacked and fastened to each other. The structure of a unit cell is described with reference to FIG. 2. The unit cell includes a Membrane-Electrode Assembly (MEA) 24, in which gas diffusion layers of an anode 22 and a cathode side 23 for diffusing gas are joined to both sides of an electrolytic membrane 21, and separators 25, which have flow paths for the fuel gas and oxygen-containing gas in the anode 22 and the cathode 23, assembled to come into close contact with both sides of the MEA 24, and current collectors 26 and 27 disposed on both sides of the separators 25 to collect current for the anode 22 and the cathode 23.
The electrolytic membrane 21 of the MEA 24 is an ion-exchangeable polymer membrane. A representative electrolytic membrane 21, which is commercialized, includes a Nafion membrane from DuPont. The electrolytic membrane 21 functions to interrupt the cross-over of oxygen and hydrogen while functioning as a hydrogen ion transporter. The anode 22 and the cathode 23 catalysts which consist of Platinum(Pt) and carbons for the active site and supporting materials respectively, are each constructed so that porous carbon paper or carbon cloth is joined to both sides of the electrolytic membrane 21.
Each of the separators 25 is made of a compact carbon plate and has a number of ribs formed thereon, thus forming fuel gas flow path grooves 31 along the surface of the anode 22 and forming oxygen-containing gas flow path grooves 32 along the surface of the cathode 23 when the separators are assembled.
On the stack 10, a temperature detection sensor 40 is installed as a temperature detection means for detecting the temperature inside the stack 10, and a heater 50 is installed as a heating means for heating the stack 10 when the measured value detected by the temperature detection sensor 40 is below a lower limit of a specified range.
However, basically, the reaction of the fuel cell is a exothermic reaction, so that, when only a heater is provided, it is difficult to maintain a desired temperature. In practice, when the fuel cell is operated within a range of high current, a temperature difference occurs even between the anode and the cathode side.
Further, in the case of heating using a cartridge-type heater, it is impossible to heat uniformly over the entire cell area, and thus temperature differences occur in some portions.
When a humidifier and a pipe between the humidifier and the cell are heated in order to operate the cell, especially in high relative humidity, a gas containing high-temperature vapor directly flows into the cell, and thus the temperature of the cell can be increased.
In addition, since the saturated vapor pressure of water rapidly changes around a temperature of about 60° C., the small temperature differences in the cell in this condition greatly influence the performance of the fuel cell. Therefore, there is a limitation on obtaining precise data about the performance characteristics of the fuel cell.
Moreover, a conventional apparatus for evaluating the performance of a fuel cell cannot evaluate the operation characteristics in sub-zero temperatures, which has recently been an important issue in the fuel cell field. In order to evaluate the operation characteristics in sub-zero temperatures, a large-sized and expensive environmental chamber has typically been used.